A step-up transformer takes AC input and gives AC output. Then, how does the E.H.T (Extra high-tension transformer) create 18 KiloVolt or more, DC-output on the CRT (enough to shot the electrons on screen) from the 12V rectified DC output from main power-transformer?

If I assume, the television EHT is just an ordinary step-up transformer, then how does so-many kilovolts output are rectified to DC? with any special-type of diodes?


The rectifier diode is built into the flyback transformer. Then the pulsating output is smoothed into DC by the giant capacitor formed by the CRT itself. You can't typically SEE the diode because it is inside the potted secondary winding and high-tension cable exit.

Television CRTs typically have a conductive coating ("Aquadag") on the INSIDE which accelerates the electron beam towards the phosphor coating on the face of the CRT. But there is ANOTHER coating of Aquadag on the OUTSIDE which is typically connected to circuit ground. The conductive surface on the INSIDE, and the conductive surface on the OUTSIDE form a giant capacitor with the glass tube envelope as the dielectric between the capacitor plates.

For this reason, most CRTs are significant shock hazards even after the power is turned off and the connections are removed. It is recommended to discharge the CRT before handling it or working on adjacent circuits.

  • \$\begingroup\$ That means there is a cover of conductive "aquadag" on the external surface of CRT? i.e. after switch off, the undischarged circuit gives shock not-ONLY at connectors or joints, but also throughout the surface of CRT? \$\endgroup\$ – Always Confused Jun 29 '16 at 9:06
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    \$\begingroup\$ A shock generally requires touching both sides of the circuit. So in the case of the disconnected CRT that would mean the conductive outer coating and a terminal connected to the conductive inner coating. \$\endgroup\$ – Peter Green Jun 29 '16 at 11:44

The television EHT is an ordinary flyback transformer, one of the two common types of transformer we meet in engineering.

The 12v main power DC is not simply put across its input, but is chopped by semiconductor switches.

The EHT transformer will have a much higher ratio of turns than say the flyback in a switch mode power supply.

The output diode is special in some sense, in that it is built to withstand working with many 10s of kV. The limit for a practical single silicon junction diode is somewhere between 1kV and 2kV. The EHT diodes used in a TV are a series stack of many diode die placed in series, and then sealed in the same package. If you do manage to get your hands on an EHT diode (they tend to be potted inside the EHT transformer for insulation and cost), then when you measure the forward voltage drop you will find it's often well above 10v, the result of it being many individual diodes inside.

  • \$\begingroup\$ That means the dc which stepped-up, is a pulsating DC ("chopped" mentioned). so the changing magnetic field on primary coil could induce current on secondary. is that? \$\endgroup\$ – Always Confused Jun 29 '16 at 6:01
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    \$\begingroup\$ @AlwaysConfused yes, hopefully NoLongerConfused \$\endgroup\$ – Neil_UK Jun 29 '16 at 6:12

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